This invention relates generally to pelleting mills for pelletizing particulate materials and more particularly to an apparatus which permits rapid die changes whenever such changes are required due to changes in feed materials or in pellet size requirements.
The performance of a pellet mill is dictated to a great extent by the geometric configuration of the holes in the die in which the pellets are formed. The number, diameter, and length of the holes are important factors in die performance, for a given type of feed material. Many mills use a single die regardless of the material being pelletized; because by so doing they avoid the costs of a large inventory of dies as well as the time required for die changes.
This, however, results in compromising the quality and quantity of pellets produced in the mill, e.g., for animal feeds, if the die holes are too long for a given feed, the capacity of the pellet mill can be significantly reduced. Moreover, the excess hole length may require that feed conditioning temperatures be reduced which, in turn, reduces the sterilization and gelatinization actions which should take place during the conditioning and pelletizing processes. If the hole is too short, the pellets may have low durability due to inadequate compression and gelatinization. Neither situation is acceptable in production of high quality feed pellets, and the result is downgrading of the pellets so produced.
To avoid such compromises of quality, pellet mill operators have resorted to quick die change pellet mills. One such mill, referred to as a rapid die clamping mill, reduces the time required for releasing and reclamping the dies during replacement. Conventional material handling equipment such as overhead hoists, hydraulic jacks, and wheeled carts are used in this type of system. Although these aids reduce operator physical effort requirements, they provide no assistance in alignment of the dies, and they do not prevent cocking and wedging of the die on the precision fitted mating surfaces. Such systems also incorporate a multiplicity of precision parts which, being permanently attached to the pellet mill, are subject to excessive wear and corrosion damage. Finally, these systems do not improve roller changing ease or time. This is a drawback because it is often required to change rollers with the dies due to matching wear patterns developed between the dies and rollers during operation. Operation of mixed roller/die sets results in premature roller/die wear and failure. As a result, the rapid die clamping mill often does not provide adequate savings of time and improvement of performance to justify the additional cost entailed.
Another system provides a main shaft/quill shaft roller/die cartridge which can be removed and replaced as a unit. This has the advantage of rapid changeover together with retention of the dies and rollers as matched sets. Despite these valuable advantages, there are several real drawbacks to the main shaft/quill shaft cartridge system including cost, mass, size, risk of accidents, and alignment of the cartridge with the mill housing and drive unit.
Each die in this system requires a cartridge including a die, rollers, die clamps, main shaft, quill shaft, front roller support, cone, and deflectors. For mills requiring several die specifications, the cost of the several cartridges becomes a major drawback of this system. The mass of a complete cartridge, especially with the large dies now in use, becomes very large. This large mass requires heavy duty materials handling equipment for transporting, installing, and removing cartridges during die changes. The limited work space around the pelletizing mills found in most feed mills cannot accommodate this heavy duty equipment and leads to employment of combinations of smaller handling equipment which may contribute to accidental damage to the mill and injury to operating personnel. Carts, which would normally be preferred for moving dies about in the feed mill, may become unstable when loaded with a main shaft/quill shaft roller/die cartridge. This is attributable to the overhang of the extended main shaft and the consequent displacement of the center of gravity of the cart/cartridge couple to a point of marginal stability, the risks of which are readily appreciated.
In addition to the drawbacks already discussed, the main shaft/quill shaft cartridge does not satisfactorily provide for ease of alignment of the precision fitted surfaces of the cartridge with the mating surfaces of the pellet mill housing and drive unit. These mating surfaces are within the mill, so that they are not visible once the cartridge is positioned in front of the mill during installation. This leads to a "push and hope" approach to cartridge insertion which may result in damage to precision fitted surfaces and to jamming of mating parts in a misaligned orientation. The alignment criticality requires precision multi-axis adjustment as well as elevation and traverse capability for the cart. All alignment parameters for current die change systems are referenced from the floor in front of the pellet mill which, due to wear and other damage, may be unreliable.
The operator must also be skilled in alignment techniques for cartridge installation in order to avoid damage and downtime caused by misalignment and jamming. This introduces an additional element of operator sensitivity to the performance of the system and results in unacceptable variability.
Finally, long running times without die changes sometimes cause exhaustion of the lubricant between the mating surfaces of the cartridge and pellet mill. This can result in running dry and in bonding of the mating surfaces together under vibratory loading conditions. The result is extreme difficulty in separating the cartridge from the pellet mill. Design of mating surfaces with tapers to prevent sticking requires high axial clamping forces in order to maintain secure contact between the surfaces. Any loosening of the clamping force during operation causes rapid wear of the tapered surfaces and of the keys and keyways.
Thus, although it is desirable to employ a die which is precisely suited to the feed material being pelletized, the costs, risks, and difficulties attendant upon such a practice make it less attractive. As a result, some mills are forced to operate at a less than optimum efficiency and to produce pellets of inferior quality.
The foregoing illustrates limitations known to exist in present die changing systems for pellet mills. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.